DECARBONATION-REHYDRATION CHARACTERISTICS OF SALEM MAGNESITE UNDER EQUILIBRIUM CONDITION

Transcription

1 DECARBONATION-REHYDRATION CHARACTERISTICS OF SALEM MAGNESITE UNDER EQUILIBRIUM CONDITION 1 Jagram Singh, 2 Saikat Maitra, 1 Tapan Kumar Parya 1 Dept. of Chemical Technology (Ceramic Engg. Div.), University College of Technology, Calcutta University, Kolkata, India 2 Govt. College of Engg. & Ceramic Technology, Kolkata, India Abstract Decarbonation-rehydration study of naturally occurring magnesite rock carries immense practical significance in preparing different magnesia products, predicting the structure property relationship of the calcined magnesia for its effective utilization and controlling the drying-firing schedule of magnesite based ceramics. This paper presents a comprehensive thermal decarbonation - rehydration behavior of natural magnesite rock fromsalem in powder form in relation to variation of temperature of thermal treatment and relative humidity of the environment. Keyword Magnesite, heat-treatment, decarbonation-rehydration, relative humidity. 1. INTRODUCTION The decarbonation-rehydration behavior of naturally occurring ceramic mineral such as calcite, magnesite and dolomite on progressive heat treatment provides a powerful tool in ascertaining the degree of reversibility of decarbonation -rehydration reaction, changes in crystallographic forms, grain morphology and reactivity of dehydrated or rehydrated products against temperature of heat-treatment and relative humidity of the environment. Magnesite (MgCO3), Brunerite (MgCO3 +FeCO3), hydro-magnesite [Mg(CO3)2.4 H2O], brucite [Mg(OH)2] are the major magnesite bearing rocks in which MgO is the main constituent. The mineral magnesite consists essentially of magnesium carbonate crystals. Crystallographically, pure magnesite belongs to the trigonal system containing 47.82% of magnesia (MgO) and 52.18% carbon dioxide (CO2). Pure magnesite is rarely found in nature and the natural mineral tends to occur part way along an isomorphic series. Main types of magnesite are formed in different ways as follows: (i) Magnesite as a sedimentary rock. (ii) Magnesite as an alteration of serpentine. (iii) Magnesite as a vein filling. (iv) Magnesite as a replacement of limestone and dolomite. There are three commercially important forms of magnesite namely amorphous, cryptocrystalline and crystalline. Microscopically,cryptocrystalline variety shows granular texture with more or less uniform grain size. Cleavage is absent within the smaller grains, which often show conchoidal fracture with pearly luster. Talc and serpentine is the accessory impurities associated with cryptocrystalline magnesite. On the other hand, the crystalline variety contains coarse grains of magnesite with calcite, dolomite and talc present as accessory minerals and siderite in the form of solid solution. Crystal structure of magnesite has been similar to calcite which was first studied by Bragg (1914) and later by Chessin, Hamilton and Post (1965). The crystal structure of synthetic pure magnesite has recently been studied by OH et al [1] using X-ray and infrared methods. The cell dimensions obtained for magnesite are a = and c = belongingin same space group as calcite, R3C. The only variable positional parameter is the X- coordinate of Oxygen. The atomic arrangement, interatomic distances and bond angles in magnesite are presented in fig 1 and table 1 respectively. Figure. 1: Crystal structure of natural magnesite Table-1: Interatomic distances and bond angles in Magnesite Bond type Bond length (Å) Bond type Bond angle (degree) M O O1 M C O O1 M O6 O1 O M O1 O Magnesite is very important rock for making magnesium as well as basic periclase refractory bricks used as linings of steel-making furnaces. Besides, it finds widespread application in building, chemical, rubber, glass and other industrial fields such as for making sorel cement, for heat preservation, heat shielding, sound insulation in buildings. In chemical industry it is extensively used as mordant, desiccant, dissolution agent, decolorizing agent, neutralizing agent and absorbent. In addition, it is also 43 P a g e

2 used in the manufacture of artificial fibers, fertilizers, plastics and cosmetics. In India, good variety magnesite is found in Salem (TamilNadu), Almorah (Uttarakhand), Himachal Pradesh with an estimated reserve of 220 million tonnes. Magnesite rock is the most important source of MgO. Face- centered cubic lattice of MgO has very high melting point, high thermomechanical and excellent chemical durability properties. Magnesite bearing rock decomposes into MgO and CO2 when calcined at oCwhich may be regarded as an important solid-state decomposition reaction. The MgO thus formed behaves as a very reactive phase and very sensitive to moisture hydration. Decomposition is accompanied by initial loss of hydroxyl groups in form of water followed by ultimate collapse of carbonate lattice structure with expulsion of CO2. Decomposition reaction can be expressed in the generalized form as, A (solid phase) B (solid phase) + C (gas phase) However, depending on calcinationtemperature and longer calcination time,it leads to the formation of different grades of MgO possessing different physicochemical properties. These properties are very much dependent on the mode of origin of the magnesite, the mineralogical composition of the natural magnesite, particle sizeand associated impurities. Various types of MgO includes light burnt caustic magnesia, hard burnt MgO, dead burnt magnesia and fused magnesia. Dehydration/decomposition-rehydration reactions of mineralshave attracted special attention of scientists as these reactions are likely to control the nature of precursors at high temperature of calcination. V.S.S Birchal et al.[2]studied the effect of magnesite calcination conditions on magnesia hydration. The solids were characterized with respect to their chemical and physical properties and then hydrated under different temperature and time. Magnesia activity was related to sample surface areas and corresponding hydration levels. Girgis and Girgis[3] investigated the surface and pore structure of talc magnesite which mentioned that the calcination temperature and the duration of thermal treatment are the important factors determining the surface properties of calcined MgO and hence the reactivity of MgO. K. Ebrahimi-Nasrabadi et al.[4]studied the timetemperature-transformation (TTT) diagram of caustic calcined magnesia. The conversion of cryptocrystalline magnesite to caustic magnesia was studied experimentally at temperatures of C and for calcination times of up to 8 h. It was reported that high-quality caustic calcined magnesia can be produced in a narrow temperature ( C) and time range (1 4 h). The dehydration-rehydration characteristics of cordierite alumina composite precursor derived through semicolloidal route are reported to be very much dependent on the dehydration temperature, relative humidity of the environment and alumina content of composite precursor [5]. The present investigation deals with the careful study of dehydration/decarbonation-rehydration behavior of magnesite rock of Salem deposit in powder form with respect to variation of heat treatment temperature and relative humidity of the environment. 2. EXPERIMENTAL Naturally occurring hard magnesite rock of Salem (Tamil Nadu) has been chosen as starting material for the study. The bulk sample in lump form was at first crushed and pulverized to fine powder, sieved into ( ) B.S. mesh size fraction, dried at 110 o C and stored in anincubator for further thermal analysis directly. The finely agated sample was chemically analyzed with respect to chemical constituents following standard method of ore analysis. It was then characterized in terms of specific surface area using Strohlen Area Meter II based on the low temperature nitrogenadsorption BET principle. Average particle size was performed by Malvern Zetasizer(ZEN-1600)in the region of micrometer. Fourier Transform Infra red (FTIR) analysis of magnesite powder was recorded by a Perkin-Elmer-783 instrument in KBr phase in the frequency region cm -1 for isolating the nature of bond. For identification of crystalline phases in magnesite,x-ray diffraction(xrd) analysis of the sample were carried out using Phillips automatic X-ray diffractometer (X`Pert PRO PW-3071) with Ni filtered Cu-K radiations of Å at a scanning rate of 2.0 o min -1. The microstructure of magnesite was studied using scanning electron microscopy (SEM) (Quanta200, FEI make, Holland).The thermal behavior of the sample was examined by differential thermal analysis (TG-DSC) using NETZSCH simultaneous thermal analyzer (STA- 409) at a heating rate of 10 C. min -1. The mass loss due to decomposition of magnesite sample was recorded by taking a definite weight (0.5gm) of the magnesite powder in porcelain crucible and heat treated at different temperatures from 100 o C o C at an interval of 100 o C for 2 h in an electrically controlled muffle furnace. After measuring the decomposition loss at a particular temperature, the gain in mass due to rehydration of each heat treated sample was performed successively at 35, 55, 75 and 100% relative humidities respectively allowing equilibrium period for 24h. Since the magnitude of rehydration at a given temperature was dependent on the amount of carbon di-oxide lost during its decarbonation process, the percentage of rehydration of heat treated sample at each temperature was measured by: (R/D)* P a g e

3 where, R = mass gain due to rehydration on the basis of the decarbonated weight of the same sample after heat treatment and D = decarbonated weight. 3. RESULTS AND DISCUSSION The study on thermal decomposition of natural magnesite appears to be a useful tool in adjudging the pathways of thermal decarbonation of bivalent carbonate rocks. The physico-chemical properties of Salem magnesite sample are presented in Table 2. Table 2.Physico-chemical characteristics of Salem magnesite Chemical Unit Salem Constituent MgO wt.% CaO wt.% 0.48 Al 2 O 3 wt.% 0.35 Fe 2 O 3 wt.% 0.84 SiO 2 wt.% 3.88 LOI at wt.% o C Texture Hard, heterogeneous texture, Greenish white in appearance Loose Bulk gm/cc 1.21 Density Specific m 2 /gm Surface Area Average m 5.85 Particle Size FTIR spectral bands cm , , , 885.6, 747 Crystalline phase DSC peaks ( o C) at 10 o C/min heating rate Major: Magnesite Minor: Quartz, Siderite 150.0, (endo) Typical chemical analysis of raw Salem magnesite sample showed high MgO content as major constituent in association with SiO 2 and Fe 2 O 3 as minor constituents. The L.O.I. value of wt % is closely consistent with that of pure magnesium carbonate, thereby indicating better quality of the magnesite. Moderate loose bulk density of magnesite powder depicted its fine particle The pulverized magnesite powder exhibited somewhat low surface area of m 2 /gm and average particle size of 5.85 m. The IR active absorption in a mineral arises from the vibration of atoms, restraining forces of bonds and geometry of the structure. The nature of absorption frequencies from different bonds in the IR spectra of a mineral provides finger print for its identification. The magnesite rock exhibited a few IR active absorption bands at different wave numbers. The FTIR spectra of magnesite showed strong absorption bands at , 885.6, and 747 cm -1 respectively. The appearance of broad absorption peak at cm -1 indicated the presence of bound OH group in the molecule. A weak peak at cm -1 was assigned to the stretching vibration of Al-Cl bond. The sharp peaks at and 747 cm -1 arose from out of plane bending mode and in plane bending mode of carbonate ion respectively. The existence of sharp absorption peak at cm -1 was due to asymmetric stretching vibration of carbonates. The Powder XRD pattern of Salem magnesite showed sharp characteristic peaks of magnesite and siderite Figure. 2: FTIR spectra of Salem magnesite crystals as well as that of quartzite. The peaks shift slightly perhaps due to the existence of magnesite-siderite solid solution. 45 P a g e

4 Figure.3: Powder XRD pattern of Salem magnesite The SEM micrograph of dried Salem magnesite revealed a morphologyof elongated grains with particle size distribution in the region of micrometer. Figure.4: Secondary electron image of Salem magnesite The TG-DSC curve of raw magnesite highlighted one broad and small endotherm within150 o C due to dehydroxylation of magnesite and a major sharp endothermic peak at 620 o C in the temperature range o C out of decarbonation reaction of magnesite rock. TG curve exhibited a total mass loss of about 51% in the studied temperature range which closely matched with the theoretical value of % for decarbonation reaction of pure magnesite rock according to the following equation: MgCO 3 MgO + CO 2 Figure.5: TG-DSC curve of Salem magnesite at the heating rate of 10 degree C.min-1 A comprehensive thermal decarbonation - rehydration behavior of naturally occurring magnesite rockof Indian origin in powder form in relation to variation of heat treatment temperature and relative humidity of the environment is described below: The curve obtained by plotting equilibrium decarbonation loss against heat treatment temperature for Salem magnesite has been represented in fig6and the results are given in the table P a g e

5 % Dehydration-Decarbonation loss IJRREST Table 3: Dehydration-Decarbonation loss of Salem magnesite sample on progressive heat treatment Temperature ( C) Percent dehydrationdecarbonation loss of Salem magnesite sample The dehydration/decarbonation curve is typically sigmoid in nature. From the figure6 it is apparent that the percent weight loss due to decarbonation remains initially slow upto 300 C followed by a sharp rise at500ºc and then become almost flat upto final temperature of heat treatment (900ºC). The initial dehydration loss recorded as 1.28 % for Salem magnesite whereas mass loss of 50.90% accounts for total decarbonation loss for the magnesite sample which revealed that they are hydro magnesite in nature to certain extent and complete breakdown of carbonate structure of magnesite and removal of carbon dioxide occurs during heat treatment at around ºC and above. Rehydration of decarbonated magnesite provides an important insight in connection with the reactivity of the formed magnesia and gradual changes in itshydration capacity as MgO derived is ionic in nature possessing a strong affinity for moisture. The structural stability and surface activity of derived caustic magnesia was studied through the rehydration of dehydrated-decarbonated magnesite obtained by heat treatment at different temperatures Temperature ( o C) Figure.6: Equilibrium dehydration-decarbonation of magnesite powder on progressive heat treatment More or less similar relationship for percent rehydration verses heat treatment temperature of the sample was observed for all relative humidities though differing only in magnitudes. In Salem magnesite, the magnitude of rehydration increases initially with increasing heat treatment temperatures attains highest value of 10.25%, 13.8%, 25.7% and 56.6% at 35%, 57%, 75%, and 100% relative humidity respectively with the samples decomposed at the temperature of 500 C. At saturated humidity, the significant increase in rehydration tendency for the magnesite sample decarbonated at 500 C and above is related to the surface condensation and strong hydration affinity of derived MgO. However, Salem magnesite showed a downward trend in the rehydration tendency with the decomposed sample derived by heattreatment of 600 o C and above. Table 4: Mass gain due to rehydration of the decarbonated Salem magnesite sample at different relative humidities Temperature of heat treatment( C) at 35% RH at 57% RH at 75% RH at 100% RH 200 C C C C C C C C P a g e

6 % Rehydration gain IJRREST REFERENCES % RH 57 % RH 75 % RH 100 % RH [1] K.D. OH, H. Morikawa, S. Iwai, and H. Aoki, The Crystal Structure of Magnesite, American Mineralogist, Vol. 58, pp , [2] V.S.S Birchal, S.D.F Rocha, V.S.T Ciminelli, The Effect of Magnesite Calcination Conditions on Magnesia Hydration, Minerals Engg., Vol. 13, pp , [3] B.S. Girgis and L.G. Girgis, Surface area and Pore Structure of Talc-Magnesite, J. App. Chem., Vol. 19, No. 10, pp , [4] K. Ebrahimi-Nasrabadi, M. Barati, P.W. Scott, Time- Temperature-Transformation (TTT) diagram of Caustic Calcined Magnesia, CIM Journal, Vol.6, No. 1, pp , C 300 C 400 C 500 C 600 C 700 C 800 C 900 C -- Temperature ( o C) Figure.7: Mass gain due to rehydration of the decarbonated magnesite powder at different relative humidities From table 4, it is evident that the magnesite sample exhibited significant rehydration mass gain in between 500ºC-600ºC indicating its structural disorders and ultrareactivity of the derived products at all relative humidity from 35 to 100%. However, magnesite decarbonated at 900ºC becomes least surface reactive as observed from the low values of percent rehydration at all humidities indicating lesser degree of reversibility. 4. SUMMARY AND CONCLUSIONS Decarbonation-rehydration characteristics of magnesite rock showed that maximum decarbonation mass loss and rehydration mass gain take place at the temperature region in between C. Magnesite sample exhibited highest surface reactivity at the temperature of 500 C for all relative humidities where it exhibited maximum decarbonation mass loss as well as rehydration mass gain. The higher order of surface reactivity in the Salem magnesite heat treated at o C is probably related to the disorder structure and finer particle sizes of fully decarbonated magnesia. Salem magnesite sample becomes more or less surface inactive at 800 C and its above temperature in presence of upto 75 % relative humidity of the environment. The study generates a very useful data on the thermal behavior of natural Salem magnesite which may be immensely utilized for the preparation of active MgO precursor, ascertaining the degree of reversibility and selecting proper drying-firing schedule of magnesite based ceramic bodies in relation to heat treatment temperature and relative humidity of the environment. 48 P a g e